EP4399385B1 - Solid lubricant for zn-ni coating on a threaded tubular element - Google Patents

Description

Technical field

The invention relates to coated steel components or conduits in the oil and gas, energy or storage field, for use such as well operation or hydrocarbon transport, hydrogen transport or storage, geothermal energy or carbon capture.

Technological background

Here, the term "component" means any element, accessory or conduit used to drill or operate a well and comprising at least one connection or connector or a threaded end, and intended to be assembled by a thread to another component to form with this other component a threaded joint. The component may be, for example, a tube or a tubular threaded element of relatively great length (in particular around ten meters in length), for example a tube, or a tubular sleeve of a few tens of centimeters in length, or an accessory of these tubular elements (suspension device or "hanger", section change part or "cross-over", safety valve, connector for drill rod or "tool joint", "sub", and the like).

Tubular threaded components or elements have threaded ends. These threaded ends are complementary, allowing the connection of two male ("Pin") and female ("Box") tubular threaded elements to each other. There is therefore a male threaded end and a female threaded end. So-called premium or semi-premium threaded ends generally have at least one stop surface. A first stop can be formed by two surfaces of two threaded ends, oriented substantially radially, configured so as to be in contact with each other after the threaded ends have been screwed together or during compressive stresses. The stops generally have negative angles relative to the main axis of the connections. Also known are intermediate stops on joints having at least two thread levels. A threaded portion, a stop surface and a sealing surface may form a unit called a threaded end. There may be a threaded end with a thread facing the outside of the tube, i.e. a male threaded end, and a threaded end with a thread facing the inside of the tube, i.e. a female threaded end. Tubular threaded elements, such as casing and completion, are made of steel and may be manufactured, without limitation, in accordance with API specification 5CT or 5CRA for standard casing and tubing. For example, the steel may be one of the grades L80, P110 or Q125.

The operating conditions of these threaded tubular elements generate different types of loads. These loads have been partially reduced, among other things, by using films or greases on the sensitive parts of these components such as the threaded areas, the abutment areas or the metal/metal sealing surfaces. The stresses induced include in particular the stresses due to storage maintenance, requiring the application of storage greases (different from the top-up greases applied before commissioning). However, other solutions exist, consisting of using organic coatings. Thus, screwing operations are generally carried out under a high axial load, for example due to the weight of a tube several meters long to be connected via the threaded end, possibly aggravated by a slight misalignment of the axis of the threaded elements to be connected. This induces risks of seizure in the threaded areas and/or in the metal/metal sealing surfaces. Thus, the threaded areas as well as the metal/metal sealing surfaces are commonly coated with lubricants.

In addition, threaded tubular elements are often stored and then used in an aggressive environment. This is the case, for example, in an offshore situation in the presence of salt spray, or in an onshore situation in the presence of sand, dust and/or other pollutants. Therefore, it is necessary to use different types of anti-corrosion coatings on the surfaces that are loaded during screwing, which is the case for the threaded areas or even the areas in tightening contact, but also the case for the metal/metal sealing surfaces, the bearing surface and the stops.

However, in view of environmental standards, it appears that the use of greases conforming to API RP 5A3 (American Petroleum Institute) does not constitute a long-term solution, since such greases are likely to be extruded from tubular components and released into the environment or into the wells, causing blockages that require special cleaning operations.

An alternative to greases is to use a first layer or a dry and/or solid deposit. These metallic deposits can be applied chemically or electrochemically. Depending on the nature of the deposit, it can provide an anti-corrosive and lubricating property to prevent the tubular threaded connections from seizing during screwing, and this in a more durable manner than an applied grease and less polluting due to better solidity. However, it has been shown that these deposits can themselves be subject to corrosion, and therefore possibly to delamination, in an aggressive environment, for example in a humid environment, by aging of said deposit, during over-stressing of the connection, during its operation in a well, as well as during repeated screwing and unscrewing operations. This corrosion or delamination is undesirable because it can involve both a risk of weakening the connection and a loss of sealing by creating a leak path linked to the corrosion of the steel substrate of the tubular threaded element. A leak can cause considerable economic or even environmental consequences, for example when a leak occurs in a hydrocarbon well during its exploitation.

A state-of-the-art solution disclosed by EP 3286288 is to add a trivalent chromium passivation type conversion layer over a solid deposit in order to isolate said solid deposit. However, the applicant has determined that this passivation layer does not assume a lubricating function, nor does it improve the lubricating properties of a top layer. Due to this lack of lubricating capacity, there is a loss of effectiveness in screwing/unscrewing tests with a higher risk of seizure and alignment of the connection, as well as an undesired increase in the screwing torque. An increase in the screwing torque implies a risk of exceeding the capacities of the screwing wrench and implies an inability to screw the connection and ensure its sealing.

The state of the art WO2016170031 describes a tubular element for drilling and/or operating a hydrocarbon well, and more specifically the threaded end of an element of the type. Said end may comprise a first layer of Zinc-Nickel, a second passivation layer and finally a layer of lubricant.

The state of the art WO2001610425 describes a threaded joint for steel tubes which exhibits galling resistance and excellent airtightness without the use of compound grease.

A line is a groove or scratch.

Generally, the deposition of a conversion layer is carried out by inserting the surface of interest into a chemical bath for which the parameters are controlled. such as the duration of deposition, the composition and temperature of the chemical solution.

The invention makes it possible to solve all of the problems mentioned above. In particular, the invention proposes to improve and stabilize the underlayer or deposit solid, while presenting a conversion treatment compatible with current equipment, a chemistry as well as easily controllable bath management.

According to one embodiment, the invention provides a tubular threaded element for drilling, exploitation of hydrocarbon wells, transport of oil and gas, transport or storage of hydrogen, carbon capture or geothermal energy, comprising a metallic body and comprising a metallic body and at least one threaded end comprising at least one threaded portion, said threaded end comprising a multi-layer coating on at least a portion of the surface of the threaded end characterized in that said multi-layer coating comprises a first layer comprising a solid coating comprising Zinc-Nickel electrodeposited on said at least a portion of the surface of the threaded end, a second oxalation type conversion layer above the first layer, a third layer comprising a polyurethane or epoxy matrix loaded with lubricating solid particles above the second layer.

Thanks to this characteristic, the first layer of solid deposit comprising zinc-nickel provides improved lubricating properties, the effects of which are protected from delamination, lineage and seizure. Indeed, the second oxalation layer acts on the first layer as a solid lubricant. The latter makes it possible to give the layer of solid deposit comprising Zn-Ni a much more stable and durable coefficient of friction. Said coefficient of friction obtained may be less than 0.2. Indeed, beyond 0.2 the applicant has noted that there may be a risk of seizure. The second oxalation layer also makes it possible to provide a chemically or mechanically insulating barrier effect for the sub-layer or first layer of solid deposit comprising Zn-Ni. The second oxalation layer amplifies the effects of the third lubricating layer. The latter provides an additional lubrication effect, thus increasing the screwing capabilities of a connection.

Surprisingly, the conversion layer, when it is of the oxalation type, has a homogeneous appearance around the circumference of the connection, regardless of the flows and temperatures in the bath. This makes it much easier to identify a coating problem. In addition, the oxalation layer allows for better adhesion of the entire coating in wet conditions and after aging, i.e. better performance retention even after long storage.

Furthermore, the use of oxalic acid is less restrictive with regard to current regulations and is not a CMR classified product, i.e. it is not carcinogenic, mutagenic or reprotoxic.

A covering problem is understood to mean a lack of coverage of the undercoat or the first deposit layer, i.e. areas of the first layer not covered by the oxalation layer and visible to the naked eye.

According to one embodiment, the tubular threaded element has a second oxalation-type conversion layer which may comprise nickel oxalate and/or zinc oxalate.

Thanks to this characteristic, nickel oxalate and zinc oxalate can delay metal-to-metal contact and store part of the energy dissipation emitted during the screwing of the connection. Surprisingly, it has been found that the addition of nickel oxalate improves the anti-corrosion performance of the conversion layer.

According to one embodiment, the tubular threaded element has a second oxalation type conversion layer which may comprise 10 to 20% carbon, 35% to 50% zinc and 35 to 45% oxygen, 0% to 35% nickel.

Thanks to this characteristic, surprisingly it was determined by the applicant that the oxalation layer gives better material resistance to the multi-layer coating.

According to one embodiment, the layer weight of the second layer may be between 0.1g/m ² and 20g/m ² .

Thanks to this characteristic, it has been determined that endurance is proportional to the weight of the layer, the greater the weight of the layer, the more endurance one has.

On the other hand, when the layer weight exceeds a certain threshold, we end up with cohesive failure problems in the oxalation layer. The layer ends up breaking on its own when subjected to external stresses. Consequently, there will be risks of delamination and flaking for both the oxalation layer, which will lead to delamination of the third layer.

According to one embodiment, the surface mass of the second layer may be between 0.5g/m ² and 10g/m ² . The applicant has determined that up to 10 g/m ² we have a better compromise between good endurance and reduced risk of cohesive rupture.

According to one embodiment, the porosity of the second layer may be between 5% and 35%.

According to one embodiment, the porosity of the second layer may be between 10% and 25%.

Thanks to this characteristic, the porosity allows to improve the retention and the coating of the upper layer thanks to a phenomenon of anchoring of the upper layer in the empty spaces of the oxalation layer.

According to one embodiment, the thickness of the second layer may be between 0.5 µm and 30 µm.

According to one embodiment, the thickness of the second layer may be between 1 µm and 20 µm.

This feature improves the resistance of the multi-layer coating material. When the layer thickness exceeds 30 µm, cohesive failure problems may occur. A layer less than 0.5 µm may be insufficient and cause problems with insufficient lubrication.

According to one embodiment, the second layer may comprise a microcracked polyhedron type texture with edges from 1 µm to 30 µm in width.

A microcracked polyhedron is a 3-dimensional geometric shape with polygonal plane faces that are grouped into segments called edges. The number of faces and edges is random, and the edge length can range from 0.5 µm to 30 µm. The layer may have randomly distributed microcracks. The crack width can range from 0.05 µm to 1 µm wide.

Thanks to this feature, a micro-cracked polyhedron type texture gives the upper layer an improved capacity for retention and adhesion to the oxalation layer.

• Une étape d'électrodéposition d'une couche de zinc-nickel sur une surface métallique d'une extrémité filetée
• Une étape de conversion de type oxalatation par immersion
• Une étape de recouvrement par une couche lubrifiante comprenant une matrice polyuréthane ou époxy chargée en particules solides lubrifiantes

According to one embodiment, the invention is also a method of manufacturing a tubular threaded element comprising the following steps:

• A step of electroplating a zinc-nickel layer onto a metal surface of a threaded end
• An oxalation-type conversion step by immersion
• A step of covering with a lubricating layer comprising a polyurethane or epoxy matrix loaded with lubricating solid particles

Thanks to this characteristic, the coating is produced without visible alteration of the Zinc-nickel layer.

Immersion is a treatment technique involving immersing the surface in an oxalation bath.

According to one embodiment, the oxalation type conversion step can be carried out at a temperature between 25°C and 90°C.

Thanks to this feature, it is possible to use the same tools as those for passivation, less expensive to set up and in material.

According to one embodiment, the oxalation type conversion step may comprise the use of an oxalic acid and the concentration of said oxalic acid may be between 1 g/L and 75 g/L.

This feature allows for better management and control of the oxalation layer weight. Indeed, the surface conversion reaction is faster as it approaches 75 g/L.

According to one embodiment, the oxalation type conversion step may comprise the use of an oxalic acid associated with an additive chosen from a nitrate, chloride, thiocyanate, or thiosulfate element or several additives in combination.

Thanks to this characteristic, an additive makes it possible to accelerate the surface conversion reaction, and thus to arrive more quickly at the desired layer weight characteristics.

The oxalation layer deposition process can be carried out over a time range of 30 seconds to 15 minutes. Indeed, time has an influence on the layer weight value. This value is proportional to the immersion time.

Below 30 seconds the layer weight will be insufficient. Above 15 minutes there is no significant change in the layer weight value.

Brief description of the figures

The invention will be better understood, and other objects, details, characteristics and advantages thereof will appear more clearly during the following description of several particular embodiments of the invention, given solely for illustrative and non-limiting purposes, with reference to the accompanying drawings.

There Figure 1 shows schematically, in a partial longitudinal sectional view, a joint resulting from the assembly of two male and female tubular threaded elements according to the invention.

There Figure 2 shows schematically, in a longitudinal sectional view, a section of the multilayer coating according to the invention.

There Figure 3 shows a graph representing the evolution of the screwing torque at each end of screwing for different types of coatings on tubular threaded elements, for different connections including either oxalation or passivation.

There Figure 4 shows a graph representing the evolution of the screwing torque at each end of screwing for different types of coatings on tubular threaded elements, for one of the connections different from that of the Figure 3 and including either oxalation or passivation.

There Figure 5 shows a graph representing the number of steps required in a BOWDEN test versus layer weight for different types of conversion, particularly to achieve 0.2 coefficient of friction versus layer weight.

There Figure 6 shows an image taken by SEM (Scanning Electron Microscope) observation, in a longitudinal sectional view, of a multilayer coating according to the invention.

There Figure 7 shows an image taken by SEM (Scanning Electron Microscope) observation, according to a longitudinal sectional view, of a multi-layer coating according to the state of the art.

There figure 8 shows an image taken by SEM (Scanning Electron Microscope) observation according to a top view of the surface of an oxalation-type conversion layer according to the invention at x5000 magnification.

There figure 9 describes an image taken by SEM (Scanning Electron Microscope) observation according to a height view of the surface of an oxalation type conversion layer according to the invention at x20000 magnification.

Description of the embodiments

In the remainder of the description, the terms "longitudinal", "transverse", "vertical", "front", "rear", "left" and "right" are defined according to a usual orthogonal reference as shown in the drawings, which includes:

A longitudinal X axis, horizontal and from left to right of the sectional views;

Furthermore, in the description and claims, the terms "external" or "internal" and the orientations "axial" and "radial" will be used to designate, according to the definitions given in the description, elements of the tubular threaded connection. The longitudinal axis X determines the "axial" orientation. The "radial" orientation is directed orthogonally to the longitudinal axis X.

There Figure 1 shows a joint or connection, along the longitudinal axis X, of a first male tubular threaded element (1) according to the invention, comprising a metal body (5) and a male threaded end (3), said male threaded end (3) comprising a male abutment surface (6), a male sealing surface (8) and a male threaded portion (14), the first male tubular threaded element (1) being shown assembled with a second tubular threaded element (2) according to the invention, comprising a metal body (5) and a female threaded end (4), said female threaded end (4) comprising a female abutment surface (7), a female sealing surface (9), and a female threaded portion (15).

Each of the male (3) and female (4) threaded ends are constituted by a metal substrate (20) and a multi-layer coating (10) on this metal substrate (20).

The tubular threaded elements (1, 2) are shown in the screwed state but the invention does not exclude that they may be in the unitary, unscrewed state.

The multi-layer coating (10) may be on either of the male (3) and female (4) threaded ends or on both at the same time. In particular, said multi-layer coating (10) may be on a male (6) or female (7) abutment surface, on a male (8) or female (9) sealing surface, or on a male (14) or female (15) threaded portion, on several of these surfaces or all of these surfaces. Regarding the Figure 1 , the multi-layer coating (10) is on the male threaded end (3).

There Figure 2 shows a longitudinal sectional view of a multilayer coating (10) on the metal substrate (20) of a male threaded end (3). However, said coating (10) can just as easily be on the metal substrate of a female threaded end. Thus, all of the developments concerning the multilayer coating (10) on the male threaded end (3) apply analogously to a multilayer coating (10) on the female threaded end.

In particular, the figure shows the multi-layer coating (10) comprising a first layer (11) of a solid coating comprising Zinc-Nickel electrodeposited on the surface of the male threaded end (3), i.e. on the metal substrate (20) which constitutes said male threaded end (3).

Said multilayer coating (10) comprises a second oxalation-type conversion layer (12) above the first layer (11).

The second conversion layer (12) of the oxalation type may comprise nickel oxalate and/or zinc oxalate (not visible in the figure). These two elements may come from the oxalation layer, by reaction between the oxalic acid and the Zinc-nickel layer.

Finally, a third lubricating layer (13) comprising a polyurethane or epoxy matrix loaded with lubricating solid particles is deposited above the second layer (12). The lubricating solid particles are selected, in a non-limiting manner, from PTFE, talc, chromium oxide, alumina.

Advantageously, the first layer (11) of solid deposit comprising zinc-nickel provides improved lubricating properties, the effects of which are protected from delamination, lineage and seizure. Indeed, the second oxalation layer acts on the first layer as a solid lubricant. The latter makes it possible to give the first layer of solid deposit (11) comprising Zn-Ni a much more stable and durable coefficient of friction. Said coefficient of friction being less than 0.2. Indeed, beyond 0.2 there is a risk of seizure.

The applicant has in fact demonstrated by comparison that this stability and durability were not found with a passivation layer (see fig. 3 and fig. 4 ). The second oxalation layer (12) also makes it possible to provide a chemical or mechanical insulating barrier effect for the sub-layer or first layer (11) of solid deposit comprising Zn-Ni. The second oxalation layer (12) amplifies the effects provided by the third layer (13). The latter provides an additional lubrication effect in synergy with the lubricating effect provided by the second layer (12), thus increasing the screwing capacities of a connection (see fig 3 ).

Furthermore, the use of oxalic acid is less restrictive with regard to current regulations and is not classified as CMR, i.e. it is not carcinogenic, mutagenic or reprotoxic.

Advantageously, the second oxalation layer (12) comprises nickel oxalate and zinc oxalate and makes it possible to delay the metal/metal contact and to store part of the energy dissipation emitted during the screwing of the connection.

Indeed, when the coating (10) is crushed under the action of screwing the threaded ends, the functional surfaces of the ends come into contact with very high contact pressures. The second layer (12) will be crushed first, undergoing the effects of stresses and pressures before the first solid layer (11), which preserves said first layer (11) and improves the overall endurance of the coating (10). Moreover, surprisingly, it has been found that the addition of a nickel oxalate makes it possible to improve the anti-corrosion resistance of the conversion layer.

There Figure 3 shows, in a comparative manner, the evolution of the screwing torque at each end of screwing for different types of coatings on tubular threaded elements, either according to the state of the art with a passivation type layer, or according to the invention with an oxalation type layer. The end of screwing is understood to mean the moment when the two stops of a male tubular threaded element and a female tubular threaded element are in contact during a screwing/unscrewing cycle (M&B).

One method used to ensure connections are properly assembled and to determine when to complete tightening is to monitor the torque applied by a collet chuck versus the number of turns. A collet chuck is a high-capacity, self-locking wrench used to grip male and female components of the connection and apply tightening/loosening torque. By connecting a computer to the load cell on the clamp and an electronic revolution counter, a graph can be plotted showing the torque on the vertical axis and the number of revolutions on the horizontal axis. By collecting each of the screwing finishes carried out, a new graph can be plotted as represented by the Figure 3 .

There Figure 3 also shows a broken line located at approximately 70,000 Nm which represents the PLT, i.e. the maximum wrench capacity. When the curve approaches or reaches this maximum wrench capacity, there is a high probability that the connection will seize and limit the maximum number of tightening/loosening or screwing/unscrewing (M&B) possibilities.

All connections used in this Figure 3 , regardless of the coating, are identical, i.e. they correspond to VAM ^® SLIJ-II type tubular threaded elements. These types of tubes are tested and validated according to API RP 5C5:2017 CAL II.

Each curve represents a coating comprising a first layer of electroplated zinc-nickel, as well as a second conversion layer of passivation or oxalation type according to the invention and a third lubricating layer. Therefore, only the nature of the second conversion layer varies from one curve to another.

Curves 1 and 2 represent a coating including a chromium III passivation layer. The presence of two curves corresponds to two screwing tests with an identical coating.

Curves 3 and 4 represent a coating comprising an oxalation layer for which an iron nitrate accelerator was used during the deposition of the layer. The presence of two curves corresponds to two screwing tests with an identical coating.

Curves 5 and 6 represent a coating comprising an oxalation layer without accelerator. The presence of two curves corresponds to two screwing tests with an identical coating.

There Figure 3 shows that all the curves representing passivation, namely curves 1 and 2, show an increase in torque for each screwing/unscrewing operation and dangerously approach the PLT as the screwing/unscrewing turns progress. The opposite is observed with the representative curves of oxalation, namely curves 3, 4, 5 and 6. Indeed, these curves are substantially flat, reflecting stable torques as the screwing/unscrewing operations progress. Although curves 3, 4, 5 and 6 show stability up to 5 screwing/unscrewing turns, the applicant was able to demonstrate that this stability can go up to 15 screwing/unscrewing turns when the second layer (12) is an oxalation layer according to the invention.

In addition, for the coatings of curves 1 and 2, as the screwing/unscrewing turns progressed, the appearance of seizure, alignment at the level of the thread roots, the thread crest and the thread span, and therefore damage to the layer of electroplated Zinc-nickel was observed.

With regard to the oxalation type coatings, the applicant noted no form of seizure, no damage to the first zinc-nickel layer and the formation of a tribofilm which gives insulating barrier effects to the entire multi-layer coating.

The results of the comparative analysis are not limited and remain valid for any type of tubes in the field of oil and gas, energy or storage, for use such as well operation or transport of hydrocarbons, transport or storage of hydrogen, geothermal energy or carbon capture.

There Figure 4 described in a similar way to the Figure 3 , a comparison of the evolution of the screwing torque at the end of each screwing in the same way as for the tests of the Figure 3 , and for different types of coatings, on another type of connection, namely VAM ^® SLIJ-III.

Curves 1, 2 and 3 correspond to threaded ends coated with a coating comprising a second passivation-type layer. The coating is located on the entire threaded end, namely the thread or a threaded portion, the abutment surface and the sealing surface. There are 3 curves because they correspond to the number of tests carried out with the same coating. Curves 4 and 5 correspond to threaded ends with a multilayer coating according to the invention comprising a second oxalation-type layer.

The comparative analysis and the resulting results are similar to those developed for the Figure 3 .

Curves 1, 2, 3 show an increase in torque from the second screwing/unscrewing. Curve 1 shows an impossibility of doing a fifth screwing/unscrewing for the corresponding joint due to seizure, while curves 4 and 5 show a stability of the torque for all screwing/unscrewing operations

The applicant demonstrates that the conclusions of the superior effects in stability and reliability of oxalation according to the invention compared to passivation are not limited to VAM ^® SLIJ-II alone and can therefore be transposed from one type of connection to another.

There Figure 5 shows a graph of the number of steps required, i.e. the number of round trips of a steel ball, versus the layer weight of a second conversion layer, in a BOWDEN test to achieve 0.2 coefficient of friction depending on the type of coatings.

Each tested sample has a coating that identically comprises at least a first layer of Zn-Ni and a layer of lubricant, the variable between the samples is the presence and/or nature of the second layer.

We find a comparison of 3 types of coatings, namely a coating without conversion layer, that is to say neither passivation nor oxalatation, the layer weight will therefore be 0 g/m ² (represented by a square on the graph). A coating with a second layer of passivation type with two examples of layer weight fixed respectively at 0.1 g/m ² and 0.15 g/m ² (represented by a circle on the graph). Indeed, for passivation, it is difficult or even impossible to find layer weight values greater than 0.2 g/m ² . Finally a multilayer coating according to the invention with a second layer of oxalatation type (represented by a triangle on the graph), numerous tests have been carried out with this type of coating with different layer weight values.

When two rough moving parts are in contact, a wear mechanism can lead to material removal with generation of debris which is the consequence of plastic deformation. The value of the coefficient of friction depends on the composition and structure of the surface, its roughness and its mechanical properties such as plasticity, ductility and surface resistance to shear stresses. In the case of a coating on a connection, the value of a coefficient of friction must be less than 0.2. Indeed, beyond 0.2 there is a risk of seizure.

In order to evaluate the lubricating properties (coefficient of friction) of the coating surface, a commercially available Bowden friction tester (Shinko Engineering Co., Ltd.) was used. In the Bowden friction tester, a ball made of steel (100CR6) was moved back and forth in a straight line on a coating formed on a steel sheet while a load was applied to the ball. The coefficient of friction was measured from the friction force and the pressure load at that time.

A commercially available steel ball made of steel (100CR6) with an outer diameter of 10 mm (Amatsuji Steel Ball Manufacturing Co., Ltd.) previously degreased is used as the steel ball in the Bowden friction test.

The steel ball is applied to the evaluated coatings and moved with a pressing load of 300 N.

There Figure 5 shows that the coating without conversion layer reaches the critical threshold of 0.2 friction before reaching 150 steps.

The coating with passivation provides significantly better lubrication than a non-converted layer and achieves the critical 0.2 coefficient of friction at about 200 steps.

The oxalation coating provides superior lubrication to the two previous types of coatings, reaching the critical threshold of 0.2 of friction coefficient between 400 and 600 steps depending on the value of the layer weight. Oxalation therefore allows the solid deposit layer comprising Zn-Ni to have a much more stable and durable friction coefficient.

Indeed, the applicant determined that the oxalate layer precisely enhances the plastic deformation of ZnNi on its surface even under high contact pressure improving atomic dislocation, grain rotation and system stability before the flaking of large blocks and the appearance of large-scale defects.

It has been observed that even with lower layer weight, oxalate layers produce a longer lasting lubricating film, chemically or physically improving lubrication efficiency.

According to a variant of the invention, the layer weight of the second layer (12) is between 0.1g/m ² and 20g/m ² .

According to another variant of the invention, the layer weight of the second layer (12) is between 0.5g/m ² and 10g/m ² .

Advantageously, it has been determined that endurance is proportional to layer weight, the greater the layer weight the more endurance one has.

On the other hand, when the layer weight exceeds a certain threshold, we end up with problems of cohesive failure in the oxalation layer. The layer ends up break by itself when subjected to external stresses. Consequently, there will be risks of delamination and flaking for the oxalation layer which will lead to delamination of the third lubricating layer. The applicant has determined that when up to 10 g/m ² there is a better compromise between good endurance and a reduced risk of cohesive failure.

There Figure 6 describes an image taken by SEM (Scanning Electron Microscope) observation, according to a longitudinal sectional view, of a multilayer coating (10) according to the invention, on the metal substrate (20) of a male threaded end (3).

The multi-layer coating (10) comprises a first layer (11) of a solid coating comprising electroplated Zinc-Nickel. Said coating also comprises an oxalation-type conversion layer 12. The third lubricating layer comprising a polyurethane or epoxy matrix loaded with lubricating solid particles is not observable in the state, and a plastic coating resin (22) is used for preparing the sample for metallographic observation. This resin (22) is only useful for obtaining the image of the Figure 6 and is therefore not part of the invention.

There Figure 7 describes an image taken by SEM (Scanning Electron Microscope) observation, according to a longitudinal sectional view, of a multilayer coating (100) comprising a passivation layer (102), according to the state of the art, on the metal substrate (120) of a male threaded end (103).

The multi-layer coating (100) comprises a first layer (101) of a solid coating comprising electroplated Zinc-Nickel. Said coating also comprises a passivation-type conversion layer (102). The third lubricating layer comprising a polyurethane or epoxy matrix loaded with lubricating solid particles is not observable in the state, and a plastic coating resin (22) is used for preparing the sample for metallographic observation. This resin (22) is only useful for obtaining the image of the Figure 7 .

Observation in the Figure 6 shows a thick, textured oxalation layer that measures about 5 µm. However, according to other observations, the oxalation layer can be between 0.5 µm and 30 µm, preferably the layer can be between 1 µm and 20 µm.

Compared to the image of the Figure 7 the second passivation-type conversion layer is simply not observable because it is less than 100 nm.

This difference in observation is important insofar as it guarantees a minimum thickness for oxalation and which remains visible. By comparison, we do not find this visibility with the passivation of the Figure 7 This thickness has advantages, such as allowing better material retention. Indeed, on the one hand, a thickness that exceeds a certain threshold, in particular 30 µm, can cause problems with cohesive failure. On the other hand, a layer less than 0.5 µm or 500 nm is insufficient and will necessarily cause problems with insufficient lubrication.

Cohesive failure is an undesirable effect that can degrade or even eliminate the effect of the second layer, the first zinc-nickel layer becoming vulnerable to the environment and induced stresses.

With regard to the cracks (24) and other cracks observed in the Zinc-Nickel layer on the Figure 6 And 7 , they are obtained by preparing samples for the needs of metallographic observation.

Advantageously, the applicant has also determined that the thickness of the second oxalation layer has an insulating barrier type effect for the first solid Zinc-Nickel layer.

There Figure 6 also shows a porosity of the second layer (12) of oxalation, said porosity of the second layer (12) can be between 5% and 35%.

According to a variant of the invention, the second layer (12) can be between 10% and 25%.

The porosity of the second layer is understood to mean empty spaces between the base of the crystals, the latter being able to cover said porosity at the level of their height. We also speak of open porosity when there are cracks which present a direct path between the Zinc-Nickel layer and the third layer comprising a polyurethane or epoxy matrix loaded with lubricating solid particles.

Advantageously, the porosity makes it possible to improve the retention of the upper layer of the multi-layer coating thanks to a phenomenon of mechanical anchoring of the upper layer in the empty spaces of the oxalation layer.

Compared to the Figure 7 , we will not find this porosity for a second passivation type conversion layer according to the state of the art. Indeed, the Applicant has determined that a passivation layer is much too thin to admit any porosity.

There figure 8 describes an image taken by MEB (Scanning Electron Microscope) observation according to a height view of the surface of a conversion layer (12) of oxalation type with a magnification of x5000.

In particular, we observe that the surface of the layer is textured by microcracked polyhedra (30).

A microcracked polyhedron is a 3-dimensional geometric shape with polygonal plane faces that are grouped into segments called edges. The number of faces and edges is random, and the edge length can range from 0.5 µm to 30 µm. The layer may have randomly distributed microcracks. The crack width can range from 0.05 µm to 1 µm wide. Thanks to this characteristic, a microcracked polyhedron type texture provides retention and adhesion capacity to the oxalation layer for the upper layer.

According to a variant of the invention, the second layer (12) can be produced with an accelerator which has the effect of accentuating the homogenization of the oxalation, and thus obtaining a finer and denser layer.

There figure 9 describes an image taken by SEM (Scanning Electron Microscope) observation according to a height view of the surface of an oxalation type conversion layer according to the invention at x20000 magnification.

The developments of the figure 8 are valid and applicable to the figure 9 .

Claims (14)

1. A tubular threaded element (1, 2) for the drilling, the operation of hydrocarbon wells, the transport of oil and gas, the transport or the storage of hydrogen, the carbon capture or the geothermal energy, comprising a metal body (5) and at least one threaded end (3, 4) comprising at least one threaded portion (14, 15), said threaded end (3, 4) comprising a multilayer coating (10) on at least one portion of the surface of the threaded end (3, 4) characterised in that said multilayer coating (10) comprises a first layer (11) comprising a solid coating comprising Zinc-Nickel electrodeposited on said at least one portion of the surface of the threaded end (3, 4), a second oxalation-type conversion layer (12) above the first layer (11), a third layer (13) comprising a polyurethane or epoxy matrix loaded with solid lubricant particles above the second layer (12).
2. The tubular threaded element (1, 2) according to claim 1, characterised in that the second oxalation-type conversion layer (12) comprises nickel oxalate and/or zinc oxalate.
3. The tubular threaded element (1, 2) according to any one of the preceding claims, characterised in that the second layer (12) has a layer weight per unit area comprised between 0.1g/m² and 20g/ m².
4. The tubular threaded element (1, 2) according to claim 3, characterised in that the second layer (12) has a layer weight per unit area of the second layer (12) comprised between 0.5g/m² and 10g/m².
5. The tubular threaded element (1, 2) according to any one of the preceding claims, characterised in that the second layer (12) has a porosity comprised between 5% and 35%.
6. The tubular threaded element (1, 2) according to claim 5, characterised in that the porosity of the second layer (12) is comprised between 10% and 25%.
7. The tubular threaded element (1, 2) according to any one of the preceding claims, characterised in that the second layer (12) has a thickness comprised between 0.5 µm and 30 µm.
8. The tubular threaded element (1, 2) according to claim 7, characterised in that the thickness of the second layer (12) is comprised between 1 µm and 20 µm.
9. The tubular threaded element (1, 2) according to any one of the preceding claims, characterised in that the second layer (12) comprises a microcracked polyhedron type texture with edges of 1 µm to 30 µm width.
10. The tubular threaded element (1, 2) according to any one of the preceding claims, wherein the threaded end (3, 4) further comprises at least one stop surface (6, 7) and at least one sealing surface (8, 9), characterised in that the multilayer coating (10) covers said at least one stop surface (6, 7) and/or said at least one sealing surface (8, 9).
11. A method for manufacturing a tubular threaded element (1, 2) according to one of claims 1 to 9, characterised in that it comprises:

    - A step of electrodepositing a zinc-nickel layer on a metal surface of a threaded end (3, 4)

    - An oxalation-type conversion step

    - A step of covering with a layer comprising a polyurethane or epoxy matrix loaded with solid lubricant particles.
12. The method for manufacturing a tubular threaded element (1, 2) according to claim 11, characterised in that the oxalation-type conversion step is carried out at a temperature comprised between 25°C and 90°C,
13. The method for manufacturing a tubular threaded element (1) according to any one of claims 11 to 12, characterised in that the oxalation-type conversion step comprises the use of an oxalic acid and that the concentration of said oxalic acid is comprised between 1 g/L and 75 g/L.
14. The method for manufacturing a tubular threaded element (1, 2) according to any one of claims 11 to 13, characterised in that the oxalation-type conversion step comprises the use of an oxalic acid associated with an additive selected from a nitrate, chloride, thiocyanate or thiosulphate element or several additives in combination.

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